Electromagnetic interference, which is also referred to as radio frequency interference (RFI), is prevalent in the environment in which wireless communications networks operate and can be the primary cause of unacceptably noisy communication sessions or worse, the cause of dropped calls. Depending upon the frequency band in which the wireless communications network operates, different types of devices can be the source of such RFI. RFI can be emitted by radar devices and by wireless communications devices such as cell phones, WiFi devices such as wireless routers or wireless phones. Such RFI emissions can disrupt the operation of communication networks operating in the same frequency band as the emitted RFI. If the level of RFI rises above a particular threshold during the course of a communication session, the quality of the communication session or the ability of a wireless communication device to maintain the session can become problematical. Such RFI can manifest itself as noise or static sound as witnessed by the user of the wireless communication device or it can manifest itself as the loss of a signal to or from the wireless communication device.
A wireless communication network, such as a wireless mesh network, is typically employed in environments where it is not practical to install a wired infrastructure network. Generally speaking, a wireless, mesh network architecture is composed of a plurality of wireless routers, each of which are arranged in space to create a “cell”. A wireless mesh network is designed so that no single point of failure in the network can fatally disrupt network communications. Typically, a router in a wireless mesh network will be connected to two or more other wireless routers within range and each of the routers can incorporate some intelligence that permits it to at least temporarily re-route traffic to another router if the link over which it is currently sending traffic fails for some reason. In the event that a link fails, it is possible in a mesh network to avoid the failed mesh network link by changing the channel over which the traffic was being transmitted prior to the link failing. In this manner, the previous traffic flow between two routers can be restored with the result that the mesh network infrastructure is most efficiently utilized. More specifically, in the event that a link between two routers fails, any subsequent re-routing of traffic will have an effect on the flow of traffic through at least those routers directly linked to the router the traffic is routed away from and possibly all routers in the network. Since mesh networks are designed to provide a channel allocation scheme that optimizes the flow of traffic through the network, any single failure of a communication link between two routers may cause some or all of the other routers in the network to switch the channels over which they are transmitting and/or receiving traffic in order to re-optimize the flow of traffic through the network. As mentioned, a major cause of such communication link failure is RFI and so it is crucial to the operation of a wireless mesh network that RFI can be detected before it causes the failure of any particular communication link.
A number of different methods have been employed in wireless communications devices utilized in wireless communication networks that effectively detect the RFI mentioned above. One method for detecting RFI is to include a second transceiver in each wireless communication device that is dedicated to passively scanning the wireless medium on channels utilized by the wireless communications network. This second, dedicated transceiver maintains a listing of all channel scanned and the level of RFI on each channel. Using this approach, when unacceptable levels of RFI are experienced on a particular channel, the wireless communications device can switch to a channel that currently has an acceptable level of RFI. While the approach of including a second, dedicated transceiver in a single wireless communications works fine, the additional cost of including a second transceiver in a wireless communications device can be prohibitive.
Another method which can be employed to detect RFI is to configure two or more wireless communications devices in the same wireless communications network to periodically send a short message to each other and to use the frame lose ratio and the frame retry ratio to indirectly reflect the level of RFI that exists on a particular channel. While this approach has little impact on the bandwidth available for supporting a communications session on any of the wireless communications devices, it does not detect RFI directly and the causes for the packet lose or retry could literally be in the thousands, RFI being only one of the causes.
Another method which can be employed by a wireless communication device to detect RFI is to configure the wireless communication device to periodically switch its transceiver from the active mode of operation to the passive mode of operation and have the device monitor the different channels to determine the level of RFI on each. While this method does directly detect the level of RFI on each channel at the physical level, it adversely affects the bandwidth that would otherwise be available for communication sessions. So for example, while in the passive mode, the wireless communication device may not be able to receive or to transmit messages, which creates obvious problems.
Published United States patent application no. 2007/0147236 A1 describes a method of detecting and avoiding interference in a wireless network. Specifically, starting on page 2 in paragraph 41 is a description of a five step method for detecting and avoiding interference in a wireless network. The first step is to observe the average packet drop rate that exists in a wireless network and to store this average drop rate on a wireless communications device that uses the network. This drop rate is used as a base-line interference level. The second step is to send and receive messages during idle network times and to compare the packet drop rate with the stored packet drop rate. The third step is to observe OFDM signal activity during the Clear Channel Assessment (CCA) time or other unreserved time slot period and stores information that indicates which channel(s) have no activity. In the forth step, if the wireless device detects UWB signals during the unreserved time slot times, it determines that any excessive packet drop rate is due to interference with another UWB network and can switch to another channel. In step 5, if the wireless device does not detect any UWB signals in step 3, it determines that it is causing the interference and can switch channels.
Although the interference detection and avoidance method described above with reference to application no. 2007/0147236 A1 works fine for a communication device in a standard wireless network, this method is only applicable to an arrangement where a wireless communication device is connected to a single, wireless network access point. Further, this method is not applicable to a wireless mesh network populated by a plurality of wireless routers that operate in concert to provide the optimal path for a plurality of traffic flows through the network. Nor does this method apply to a network that has the capability to re-route traffic in an optimal fashion in the event that one or more communication links between the network routers fail. Nor does this method apply to a network that has the capability to generate, after one or more communication links have failed, a scheme for network channel usage that re-optimizes the flow of traffic through the network.